A welded joint by means of which a branching of pipe can be produced is known from Swiss Pat. No. 570,577. To produce the joint in accordance with this patent, a pipe connection resting on a saddle is placed on an existing pipe and the entire saddle is welded to the pipe by means of a heating knot. Thereafter, the pipe is drilled out in the area of the opening of the connecting pipe.
A connecting sleeve for plastic pipes which includes a wide strip bent into a cylinder is known from German Pat. No. 2,721,340. On the ends of the strip, which are adjacent to each other, the sleeve carries clamping devices by means of which it can be clamped firmly to a pipe joint. A permanent connection is produced by cementing the sleeve to the ends of the pipe. After the cement has hardened, the clamping device can be removed in order to obtain a simple form of joint.
With a welded joint made in accordance with the above Swiss patent, in which the clamping ring to be placed on an intact pipe as described therein is used, and in which complete tightness over the entire circumference of the pipe is not necessary, a gap would occur, depending upon existing tolerances, which might result in a defect in the welding if it was used to join two pipes at the adjacent ends of the heating mat. The method disclosed in the above German patent can not be used with pipelines which are subject to great mechanical, thermal or chemical stresses, since contact cementing has less resistence capability than a weld.
U.S. Pat. No. 4,096,017 shows a welded joint for plastic-coated pipes in which the pipe ends are welded together without the plastic coating in a conventional manner after which the pipe is again provided with a plastic coating by use of a plastic mat. The joint with the existing plastic coating is achieved by welding the overlapping mat by means of a resistance wire inserted between the mat and the coating. This procedure for producing a permanent coating of the welded joint connecting the pipes is not intended for use in forming a direct, tight joining of plastic pipes themselves.